A spark plug for an internal combustion engine such as an automotive engine or the like includes, for example, a center electrode extending in an axis direction of the spark plug, an insulator located on an outer side of the center electrode, a cylindrical metal shell located on an outer side of the insulator and a ground electrode joined at a rear end portion thereof to a front end portion of the metal shell. The ground electrode is bent and arranged in such a manner that a front end portion of the ground electrode faces the center electrode, thereby defining a spark gap between the center electrode and the ground electrode.
In general, the metal shell is made of an iron-based material such as low-carbon steel and coated with a nickel plating layer for improvement in corrosion resistance. For the formation of the plating layer on the metal shell, a so-called barrel plating treatment can be advantageously used in terms of productivity improvement. Herein, the joining of the metal shell and the ground electrode is commonly done by resistance welding. It is thus difficult to join the ground electrode to the metal shell when the plating layer has been applied to the surface of the metal shell. Even if the ground electrode is joined to the metal shell, a breakage may occur in the plating layer at a welded joint between the metal shell and the ground electrode and become a cause of deterioration in corrosion resistance. It is accordingly common practice to perform a plating treatment on both of the metal shell and the ground electrode, after joining the metal shell and the ground electrode together, whereby a plating film is formed over the whole of surfaces of the metal shell and the ground electrode.
However, the bending of the ground electrode toward the center electrode, with the plating film being applied to the ground electrode, can lead to separation of the plating film. When the spark plug is used in such a state that the plating film is being separated from a center-electrode-side part of the ground electrode, a spark discharge (so called “side spark”) between the separated part of the plating film and the center electrode may occur and cause deterioration in ignition performance.
It is conceivable to remove (peel off) the plating film, which has been applied to the whole surface of the ground electrode, from a given part of the ground electrode (for example, a portion of the ground electrode to be bent). There is proposed a technique of removing the plating film by immersing the given part of the ground electrode in an acidic remover while holding the metal shell with a predetermined jig. (See, for example, Patent Document 1.)